Tap changers, also termed on-load tap changers, are known devices for uninterrupted switching between winding taps of a tapped (regulating) transformer. They serve, together with the tapped transformer, for voltage regulation of public or industrial energy supply and distributing mains. Tap changers are usually actuated by a motor drive and for each switching from one winding tap to another winding tap the motor drive is actuated and, in turn, actuates selector contacts, pre-selector/coarse-selector contacts and load changeover switch contacts in a defined switching sequence specific to the equipment. Tap changers are important apparatus for fault-free energy supply, for which regular functional monitoring is useful and in many cases is even offered so as to recognize functional disturbances in good time and to be able to initiate appropriate remedies or also safety measures close in time.
A method of monitoring a tap changer is known from DE 197 44 465 [U.S. Pat. No. 6,124,726], in which during the load changeover, i.e. during the actuation of the tap changer, the effective values of current and voltage at the drive motor of the motor drive are measured and the actual power is calculated from these values and in turn the torque determined therefrom. At the same time, in this known method a positional detection of the respective instantaneous setting of the tap changer during the switching-process is carried out; as a consequence, the respectively determined torques are associated with the position in which they have occurred. These value pairs are compared with previously stored target value pairs. In the case of deviations of the actual values from the target values beyond a certain threshold, reports are generated; in the extreme case the tap changer is stopped.
A further method of that kind is known from DE 197 46 574 [U.S. Pat. No. 6,110,674]. In this known method the entire switching-sequence in the case of actuation of the load changeover switch is divided into specific regions, so-called ‘windows’; in each of these windows separate comparison of the actual values and the target values for the respective window with one another is undertaken. This method, known as ‘window technique’, is realized by the applicant in monitoring apparatus of the apparatus range TAPGUARD (Registered Trade Mark) it markets.
These known methods without exception operate retrospectively. The entire torque plot in the case of a load changeover at one winding tap to a new winding tap is initially stored. After the conclusion of the load changeover process a comparison of the corresponding actual and target values is possible. This applies just as much to the ‘window technique’. Here, too, the values comparison of the individual windows takes place only after the end of the switching process.
In other words: if during a load changeover faulty functioning occurs the known methods can indeed signal this and prevent further future damage, but the faulty functioning triggering corresponding warning signals can itself no longer be ‘undone’. In the extreme case this can lead to damage due to this faulty functioning.